In the debut of our Background Check section (“Which Light Is Greenest?” Spring 2013), David Tyler reported on a study revealing that CFL and LED light bulbs qualify as hazardous waste by U.S. EPA and California standards because of high levels of some toxic metals. One concern in the disposal of CFL bulbs is that mercury could leach into groundwater. A recent paper published in Environmental Science & Technology suggests a method for tracking mercury pollution from the bulbs. The authors report that the mercury in used CFL bulbs exhibits an isotopic signature distinct from that of other mercury sources such as coal plants. When an electric current excites the mercury vapor to set off a light-producing reaction, the mercury takes on a new isotopic composition. Because of this, the researchers suggest that isotope analysis can be used to get a more complete picture of the environmental impact of these energy-efficient bulbs. ❧